Discontinuous Turning Reaction during Escape in Soldier Crabs

Abstract

ABSTRACT In many animals, the approach of an object is perceived visually and elicits an escape reaction (Dill, 1974). The escape is often aimed directly away from the threat. In some cases, however, the animal heads towards a nearby refuge, or another preferred place. Blue crabs, for instance, escape from the shore into deeper water (Woodbury, 1986). When ghost crabs are threatened while foraging far away from their burrow, they may change their heading erratically during escape in response to the visually perceived movement of the approaching object (Vannini, 1980). Soldier crabs (Mictyris longicarpus Latreille) face a similar problem because they live on estuarine mud flats and have no permanent burrows. They may react to an approaching person at a distance of 29–33 m, i.e. when the person has a vertical angular size of 2.2–3.2° at the eye of the crab. This illustrates the high vertical visual acuity across the acute zone of the ‘flat-world type crab’ Mictyris (Zeil et al. 1986). Usually, they can avoid being caught by ‘screwing’ themselves into the substratum with astonishing speed (Cameron, 1966). On hard substrata, however, they have to escape. Unlike ghost crabs (Burrows and Hoyle, 1973), soldier crabs are slow, with a maximum velocity of about 10 cm s−1. This may be a consequence of their habit - atypical of brachyurans - of walking exclusively forwards (Sleinis and Silvey, 1980). During escape, soldier crabs walk on a straight path away from the approaching object and usually change their direction only in response to changes of the approach direction of the threatening object. I observed that when one walks around a soldier crab on a circular path at some distance, it still walks along straight paths and changes direction by making abrupt turns from time to time.